class Solution {
public:
    bool isInterleave(string s1, string s2, string s3) {
        int m = s1.size(), n = s2.size();
        if (m + n != s3.size()) return false;
        vector<vector<bool>> dp(m + 1, vector<bool>(n + 1));
        dp[0][0] = true;
        s1 = ' ' + s1, s2 = ' ' + s2, s3 = ' ' + s3;
        for (int j = 1; j <= n; j++)
            if (s2[j] == s3[j]) dp[0][j] = true;
            else break;
        for (int i = 1; i <= m; i++)
            if (s1[i] == s3[i]) dp[i][0] = true;
            else break;
        for (int i = 1; i <= m; i++)
            for (int j = 1; j <= n; j++)
                dp[i][j] = (s1[i] == s3[i + j] && dp[i - 1][j]) || (s2[j] == s3[i + j] && dp[i][j - 1]);
        return dp[m][n];
    }
};